StarDate Podcast

A star in the constellation Cetus brightens and fades dramatically every 11 months. At its brightest, it’s fairly easy to see. At its faintest, it’s visible only through a telescope. Because of that change, a 17th-century astronomer called the star Mira – from the Latin word for “wonderful.” The star changes because it pulses in and out like a beating heart. Mira’s in the final stages of its red-giant phase of life. Its core is no longer producing nuclear reactions. Instead, it’s fusing hydrogen and helium in thin shells around the core. Mira’s outer layers are puffed up by radiation from the shells. At the maximum, that inflates the star to about 400 times the diameter of the Sun. That’s also when its surface is coolest and faintest. As the outer layers cool, they fall inward, making the surface hotter and brighter. At minimum, the star is about 330 times the Sun’s diameter. Each time it puffs up, Mira loses a little of the gas at its surface. Within the next million years or so, it’s likely to expel all the gas in its outer layers. That will leave only its hot but dead core – a white dwarf. Astronomers have discovered thousands of stars like Mira. And many others will undergo the same phase, including the Sun – in about six billion years. Mira climbs into view in the east by 8:30 or 9. But it’s in the “fading” part of its cycle, so you need a telescope to see it. Script by Damond Benningfield

Algol does something amazing. Every 2.9 days, the star fades to just one-third of its usual brightness. In centuries past, the stars were thought to be unchanging. A star that changed so blatantly was a bit scary. So it was given a name to match: “Algol” comes from an Arabic phrase that means “head of the demon.” But the star’s odd behavior isn’t scary it all – Algol fades as the result of eclipses. The system consists of three stars. Two of them form a tight binary. The members of the binary orbit each other once every 2.9 days. We see the system edge-on, so the two stars eclipse each other. One star is much brighter than the other. When the fainter star crosses in front of it, the system fades dramatically. When the bright star covers up the faint one, though, the difference is tiny – much too subtle to see with the eye alone. Astronomers have cataloged hundreds of eclipsing binaries. And the eclipses are important. They reveal the relative sizes and masses of the two stars, details about their orbit, and more. So there’s nothing to fear from these up-and-down star systems. Algol is low in the northeast at nightfall, in Perseus. It should be at its brightest tonight. The faint part of its cycle will happen during daylight for the next few cycles. It’ll be visible during nighttime later in the month. Sometimes, a star can change brightness all on its own, and we’ll have more about that tomorrow. Script by Damond Benningfield

The Orionid meteor shower should be at its most active the next few nights. And there’s no Moon to get in the way, so it should be a pretty good show. The shower is named for Orion because its meteors appear to “rain” into the sky from Orion the hunter. The constellation climbs into good view after midnight, so that’s when the shower is at its best – between midnight and dawn. You don’t have to look at Orion to see the meteors, though – they can blaze across any part of the sky. The meteors are bits of debris from Comet Halley. The comet sheds grains of dust as it orbits the Sun. When Earth crosses the comet’s path, some of those grains plunge into the atmosphere. They instantly vaporize, creating the streaks of light known as meteors. Most of the dust grains are no bigger than pebbles. But a few are larger. They form brilliant streaks that are visible even in a somewhat light-polluted sky. And some of them can leave glowing trails that remain visible for a couple of minutes. The shower has been declining in recent years. Halley’s Comet is near its greatest distance from Earth, so there aren’t as many bits of comet dust in this part of its orbital path. Even so, the shower could produce 20 or more meteors per hour at its peak. To watch the Orionids, find a dark but safe site away from city lights. Bundle up against the autumn chill, then sit back and watch the sparks from Halley’s Comet. Script by Damond Benningfield

Venus doesn’t have any moons. But it does share its orbit around the Sun. Astronomers have discovered 20 asteroids known as “co-orbitals,” but there could be many more. These big space rocks follow roughly the same path as Venus. But they won’t stay in that lane forever. And when they leave it, they could threaten Earth. These objects are nudged along by the gravity of Venus and the Sun. They generally stay well ahead of or behind Venus. Only one follows exactly the same orbit as the planet. The others move in and out a bit, getting closer to the Sun, then moving farther away. Over the long term, though, their orbits aren’t stable, so they can break free and head elsewhere. A recent study found that of the 20 known objects, six could threaten Earth within the next 12,000 years. And three of them are especially dangerous. All three are at least a thousand feet in diameter, so they could cause major damage if they hit our planet. A study also found that there could be many more of these Venus groupies. They stay so close to the Sun in our sky that they’re hard to see through the solar glare. And they move quickly, making them even harder to find. But a new telescope in Chile might pick out some of them – helping us find potential threats far in advance. Look for Venus near the Moon in the dawn sky tomorrow. It’s the brilliant “morning star,” so you can’t miss it. Tomorrow: an autumn meteor shower. Script by Damond Benningfield

California is the land of the stars. It’s also in the stars as the California Nebula – a cloud of gas and dust that looks like the outline of the state. It’s more than a thousand light-years away, in Perseus. The nebula belongs to a giant star-forming complex – the Perseus O-B-2 association. The region has given birth to many class O and B stars – the biggest and brightest of all stars. The California Nebula probably is energized by one of those stars, known as Xi Persei. The star is more than 30 times the mass of the Sun, and tens of thousands of degrees hotter. At that temperature, it produces huge amounts of ultraviolet energy. When that radiation zaps hydrogen atoms, it splits them apart. When they link back up, the atoms produce red light – the main color of the nebula. Oxygen and other elements produce their own colors, but they’re not nearly as common as hydrogen. The California Nebula probably is about a hundred light-years long. It’s likely to split into smaller clumps that will collapse to form even more stars. But radiation and winds from Xi Persei and other big stars will blow away much of the nebula’s material – limiting the number of new stars for this cosmic California. Perseus climbs into good view, in the northeast, in early evening. Xi Persei is visible to the naked eye, near the bottom of the constellation. But you need a telescope to see the faint outline of the California Nebula. Script by Damond Benningfield

Xi Persei doesn’t look all that imposing. The star shines at fourth magnitude, so it’s visible under dark skies, but not from cities and towns. But that’s only because it’s a long way off – about 1200 light-years. In reality, it’s one of the most impressive stars visible to the human eye. Perseus climbs the eastern sky on autumn evenings. It consists of a couple of ribbons of stars that join at Mirfak, the constellation’s leading light. And it contains the most famous variable star in the sky: Algol, the Demon Star, which fades and brightens every three days. Yet neither can compare with Xi Persei, which is near the bottom of the longer ribbon. At visible wavelengths, it’s about 13,000 times brighter than the Sun. But it’s tens of thousands of degrees hotter than the Sun, so it emits most of its light in the ultraviolet. When you add that in, Xi Persei is a quarter of a million times the Sun’s brightness. The key to that showiness is the star’s mass – roughly 30 times the Sun’s mass. At that great heft, gravity squeezes its core tightly, revving up its nuclear engine. Energy works its way to the surface, making Xi Persei hot and bright. Before long, it’ll get even hotter and brighter. It’ll explode as a supernova, briefly shining brighter than billions of normal stars – a brilliant demise for an impressive star. Xi Persei energizes a nearby cloud of gas, and we’ll have more about that tomorrow. Script by Damond Benningfield

For decades, Regulus had astronomers fooled. The star is bright, hot, and blue – an indication that it was quite young. Most estimates put its age at no more than a hundred million years – about two percent the age of the Sun. Instead, it’s at least a billion years old. But like a vampire, it’s been rejuvenated by taking the life’s blood of a companion, making it look much younger. The star we see as Regulus is about four times the size and mass of the Sun, and more than 300 times brighter. A few decades ago, astronomers discovered its companion – a “dead” star known as a white dwarf. The two stars are so close together that the corpse was hidden in the glare of the bright star. The presence of the companion means the system has to be at least a billion years old – old enough for the companion to evolve to its present state. As it evolved, it puffed up. Gas flowed from its surface over to the other star. That made the star we see today much bigger and heavier. It also made the star hotter, which made it bluer. Hot blue stars usually are quite young. So astronomers were fooled into thinking that bright Regulus was still a youngster – not an older star that’s been rejuvenated. Look for Regulus close to the Moon at dawn tomorrow. The distance between them will narrow as you move westward. They’ll be especially close as seen from Alaska or Hawaii. Script by Damond Benningfield

The Milky Way is a giant among galaxies – a hundred thousand light-years in diameter. But a few galaxies make the Milky Way look like a mere bauble by comparison. They span millions of light-years – puffed up by the action of supermassive black holes. These monsters are known as giant radio galaxies. Not only are they large, but they produce enormous amounts of radio waves. The black hole in such a galaxy’s heart is encircled by a massive disk. As material in the disk spirals into the black hole, magnetic fields fire “jets” of some of its particles like water from a firehose. These jets can streak far into space. They end as they plow into the material between galaxies, forming “lobes” that are bright sources of radio waves. A recent study found 15 of these giants. They’re in the constellation Sculptor, which creeps low across the south on October evenings. The largest of them spans more than 12 million light-years. The galaxy itself is wider and heavier than the Milky Way. But the jets puff up its overall size. It actually has two sets of jets – one nested inside the other. The longer set is older – powered up by the black hole millions of years ago. But the black hole might have slowed down its eating for a while, shutting off that flow of particles. Later, it started chowing down again, powering the second set of jets, which continue to expand – sustaining this galactic monster. Script by Damond Benningfield

Deneb, the brightest star of Cygnus, stands high overhead as night falls at this time of year. And it really is a brilliant star – tens of thousands of times brighter than the Sun. But if we could tune our eyes to see radio waves, Deneb wouldn’t even register. Instead, the swan’s leading light would be Cygnus A – one of the brightest radio galaxies in the universe. A radio galaxy produces huge amounts of radio waves. It’s usually a large elliptical galaxy, which looks like a fat, fuzzy football. It has a supermassive black hole at its center. Gas, dust, and stars spiral into the black hole. But powerful magnetic fields eject some of that material back into space. It forms “jets” that fire out at almost the speed of light. The jets can span hundreds of thousands of light-years. Electrons spiral through a jet’s magnetic field, producing radio waves. Eventually, the jets plow into gas and dust between galaxies, forming wide bubbles that emit even more radio waves. Cygnus A was the first radio galaxy ever discovered, in 1939. It’s about 760 million light-years away. Its black hole is two and a half billion times the mass of the Sun. The entire complex – galaxy, jets, and bubbles – spans more than 600,000 light-years. That’s six times the diameter of our home galaxy, the Milky Way – one of the biggest, brightest radio galaxies in our part of the universe. More about radio galaxies tomorrow. Script by Damond Benningfield

Floating through the clouds at Jupiter’s equator sounds like a celestial carnival ride. The equator spins at about 28,000 miles per hour – 28 times faster than Earth’s equator. So the Sun, moons, and stars would zip across the sky in a hurry. Jupiter is the largest planet in the solar system – 11 times Earth’s diameter. It also spins faster than any other planet – so fast that it bulges outward at the equator. At that speed, a day on Jupiter is less than 10 hours long. So the equator always sees about five hours of daylight followed by five hours of darkness. It might not sound right, but Jupiter spins so fast because it’s so big. As it swept up more material while it was taking shape, gravity compressed it, making it smaller. The planet had to spin faster to balance the books – like a skater spinning faster as it pulls in its arms. Some studies have suggested that Jupiter might actually have been slowed down early on by its magnetic field. The young planet was encircled by a disk of gas and dust that gave birth to its moons. As the gas swirled through the magnetic field, some of it developed an electric charge. The charged-up gas grabbed on to the field, acting like a brake – slowing down the solar system’s biggest and still fastest planet. Jupiter looks like a brilliant star below the Moon at dawn tomorrow. The twin stars of Gemini are closer to the left and lower left of the Moon. Script by Damond Benningfield

If you’d like to thank your lucky stars for a bit of good fortune, we have two stars for you to look at. They’re the brightest stars of Aquarius. Both of them have names that mean “lucky.” The brighter of the two is Sadalsuud. The name comes from an Arabic phrase that means something along the lines of “luckiest of the lucky.” When the name was bestowed, the star first appeared in the dawn sky around the spring equinox. The days were getting longer and warmer, and spring rains were settling in – bringing life-giving water to the fields. So the star was considered a sign of good fortune. The other lucky star is Sadalmelik – “luck of the king.” The exact reason for its name is unclear, although it, too, may relate to the seasons. Both stars are class-G supergiants. They’re about the same temperature and color as the Sun, but much bigger, heavier, and brighter. Both stars have passed through the prime phase of life, so their luck is running out – they’re nearing the end. Each will shed its outer layers and leave behind a massive white dwarf – a corpse about as heavy as the Sun, but only as big as Earth. Aquarius is in the southeast at nightfall. The “lucky” stars line up parallel to the horizon, with Sadalmelik on the left. The stars are separated by about the width of your fist held at arm’s length. But they’re so far from us that they don’t look all that bright – a bit of bad luck for skywatchers. Script by Damond Benningfield

Stars like the Sun go through several distinct phases of life, from embryo to corpse. Consider Aldebaran, the bright eye of Taurus, which accompanies the Moon tonight. It’s more than six billion years old – older than the Sun. And it’s well into “old age.” Aldebaran was born when a cloud of gas and dust collapsed. For millions of years, it shined as a result of the heat generated by that collapse – its “embryonic” phase. Eventually, its core got hot enough to ignite the fires of nuclear fusion, and Aldebaran entered the prime phase of life – fusing hydrogen to make helium. A few hundred million years ago, it used up the hydrogen in the core. The core got smaller and hotter, and Aldebaran began fusing the hydrogen in a shell around the core. At the same time, its outer layers puffed up, so Aldebaran is more than 40 times wider than the Sun. This is the giant phase of life. Eventually, the core will get hot enough to fuse the helium to make carbon and oxygen. But when the helium is gone, fusion will stop. The core will get smaller and hotter, and its radiation will push the star’s outer layers into space. Only the hot, dead core will remain – a white dwarf. Even that isn’t the end, though. The white dwarf will cool and fade. Hundreds of billions of years from now – and perhaps much longer – it’ll stop producing any visible light at all. That will make it a black dwarf – the final stage for the eye of the bull. Script by Damond Benningfield

The Moon barrels through the Pleiades star cluster this evening. It’ll pass directly in front of the cluster, briefly blocking most of its stars from view. The Pleiades is the most famous of all star clusters. It’s also known as the Seven Sisters, but under dark skies – with no Moon in the way – you might actually see nine stars or more. But that’s only the beginning. The cluster contains more than a thousand stars. In fact, it was the first cluster to be recognized as a cluster – a group that’s moving through the galaxy together. That recognition came in 1767. John Mitchell, a clergyman and scientist, was looking at several tightly packed groups of stars. He studied the Pleiades in the greatest detail. And he calculated that there was only a one-in-500,000 chance that the grouping could be random. Instead, something had to be holding the stars together. His idea was confirmed when astronomers measured the motions of the cluster’s stars. They’re all moving in the same direction, and at the same speed. Today, we know that’s because they were born together, from a single giant complex of dust and gas. They’re bound to each other by their mutual gravitational pull. But they won’t stay together. The cluster is being pulled apart by the gravity of the rest of the galaxy. So the Pleiades probably will dissipate in about 250 million years – with its member stars going their own way. Script by Damond Benningfield

An embryonic star may be about to vanish – perhaps for a century. It’s not going anywhere. Instead, it’ll be cloaked by a dense cloud that encircles two companions. T Tauri is the prototype for a class of proto-stars. The gravity of such a star is causing it to collapse, making it hot and bright. But its core isn’t hot enough to ignite the fires of nuclear fusion, so it’s not yet a true star. The star we see as T Tauri is about twice as massive as the Sun. It’s encircled by a disk of gas and dust – the raw materials for making planets. And it might already have given birth to at least one planet. T Tauri is a member of a triple-star system. Its companion stars are close together, encircled by their own disk. It’s so thick that it hides the stars at visible wavelengths – we see them only in the infrared. Now, the companions and their disk are starting to slide between us and the brighter star. The star has faded a good bit in recent years. Eventually, it may be hidden behind the disk as well. And it could take a century for the disk to move out of the way – allowing the brightest star of the T Tauri system to shine through once more. Taurus is low in the east and southeast by late evening. T Tauri is just above Aldebaran, the bull’s brightest star, far to the lower left of the bright Moon. The young star is visible through a telescope – for now. More about the Moon and the bull tomorrow. Script by Damond Benningfield

The bull is charging into the evening sky. Taurus is in full view by about 11 o’clock, low in the east. He stands high in the south before dawn. He’s rising earlier each night, and will be in view all night long by about Thanksgiving. All the stars rise four minutes earlier each night – a result of Earth’s motion around the Sun. Earth makes one full turn on its axis against the background of distant stars every 23 hours and 56 minutes. So, if you looked at the sky every 23 hours and 56 minutes, and you could see through the daytime glare, you’d always see the same stars in the same position. But during that period, Earth moves along its orbit around the Sun. The distance it covers means the planet has to turn four extra minutes for the Sun to reach the same position in the sky. That makes a day 24 hours long. And it also means that the background stars rise and set four minutes earlier on our 24-hour clock. As a result, every star and constellation is in prime evening view at different times of the year. For Taurus, it’s fall and early winter – the time the bull charges across the evening sky. For now, look for Taurus beginning in late evening. Its brightest star is Aldebaran, the bull’s eye. His face is outlined by a V-shaped pattern of stars to the upper right of Aldebaran. And his shoulder is the sparkly little Pleiades star cluster, well above Aldebaran. More about Taurus tomorrow. Script by Damond Benningfield

The Moon is full tonight, and it’s especially bright as well. And to top things off, it’s the most famous full Moon of them all – the Harvest Moon. Harvest Moon is the full Moon closest to the fall equinox, so most years it falls in September. But once every five years or so it skips into October. This year, September’s full Moon came 15 days and 10 minutes before the equinox, which took place on the 22nd. This month’s full Moon comes 14 days, 9 hours, 29 minutes after the equinox, so it barely takes Harvest Moon honors. The Harvest Moon was important in earlier times because it shined over the fields when crops were ready to be brought in. Its light allowed farmers to work into the night. And because of the angle of the Moon’s path at this time of year, the full Moon rises only a few minutes later each night as seen from more northerly latitudes. So it’s almost like having a full Moon for several nights in a row. People often think that the Harvest Moon must be especially bright, but that isn’t usually the case. This year, however, it is. That’s because it comes less than a day and a half before the Moon is closest to Earth for its current orbit – roughly 15,000 miles closer than average. That provides some especially bright nights for farmers – and the rest of us, too. Tomorrow: the bull charges into the evening sky. Script by Damond Benningfield

It’s pretty easy to measure the length of a day on Mars or most other solid bodies. Just pick a feature on the surface and see how long it takes to spin back into view. It’s not so easy for planets that don’t have a solid surface. We can track bands of clouds, but different bands can move at different speeds. That’s been an especially tough problem for Saturn, the second-largest planet in the solar system. Scientists have been trying to pin down its rotation rate – the length of its day – for centuries. When the twin Voyager spacecraft flew past Saturn in the 1980s, they measured the planet’s magnetic field to reveal the rotation rate of its interior. But when the Cassini spacecraft orbited Saturn decades later, its observations showed the day was about six minutes longer. At the end of its mission, Cassini flew between Saturn and the inner edge of its rings. Measuring waves in the rings and tiny changes in the planet’s gravitation field produced yet another length: 10 hours, 33 minutes, and 38 seconds. That’s not necessarily the final answer. Scientists continue to study the giant planet to know how to set their Saturn clocks. And Saturn is in great view tonight. It looks like a bright star quite close to the lower right of the Moon at nightfall, and below the Moon as they set, before dawn. Script by Damond Benningfield

Saturn and Venus bracket the pre-dawn sky now. As Saturn drops from view in the west, Venus nudges into view in the east. Saturn looks like a bright star, while Venus is the brilliant morning star. The planets are both sliding eastward against the background of distant stars. Saturn lined up opposite the Sun a couple of weeks ago. For a few months around that point, the planet looks like it’s “backing up” against the background of stars – a result of the relative motions of Saturn and Earth. Earth is closer to the Sun than Saturn is, so our planet moves faster. It overtakes Saturn every 13 months, making Saturn appear to shift into reverse. It’s actually still moving in its usual direction – only our viewing angle is changing. It’s like passing another car on the highway. For a while, the other vehicle looks like it’s moving backward against the background of buildings and trees. When you move far enough past it, though, it appears to resume its normal forward motion. Saturn will end its backward motion and shift back into forward at the end of November. Venus, on the other hand, is about to pass behind the Sun as seen from Earth, so it’s dropping closer to the Sun every day. That’s also a result of the orbital motions of the two planets. Venus will disappear in the twilight in December, and cross behind the Sun in January – depriving us of the “morning star.” More about Saturn tomorrow. Script by Damond Benningfield

Scientists don’t know what dark matter is. But they have some ideas of what it isn’t. And they took a big step in ruling out some possibilities with the release of a study last year. Dark matter produces no energy – the reason it’s described as “dark.” But we know it’s there because its gravity pulls on the visible matter around it. In fact, it appears to make up about 85 percent of all the matter in the universe. The leading idea says dark matter consists of some kind of subatomic particle. A top candidate is called a WIMP – a weakly interacting massive particle. Although dark matter almost never interacts with normal matter, it might occasionally do so – ramming into the nucleus of a normal atom. That would produce a tiny spark of light, which detectors might see. One experiment is LUX-ZEPLIN. It’s in a former gold mine, almost a mile below the town of Lead, South Dakota. The rock above it blocks other types of particles from reaching the experiment. Its detectors are inside a vat filled with about 8,000 tons of liquid xenon. The hope is that a WIMP will hit a xenon molecule and trigger that spark of light. Project scientists conducted 280 days of observations. And they didn’t find any indication of WIMPs. But their test was the most sensitive yet for certain types of WIMPs. So the experiment rules out some candidate particles – narrowing the possibilities for dark matter. Script by Damond Benningfield

At first glance, the dwarf planet Ceres doesn’t seem like a friendly home for life. It’s small, dark, and scarred by impact craters. Yet a deeper look presents a more optimistic picture. It has more water than any body in the inner solar system besides Earth. It has an abundance of organic compounds – the chemical building blocks of life. And it should be warm enough below the surface to sustain microscopic life. Ceres is the largest member of the asteroid belt – a wide band of debris between the orbits of Mars and Jupiter. It’s about a quarter the diameter of the Moon. It probably consists of a dense core and mantle surrounded by an icy crust. The Dawn spacecraft studied Ceres from orbit a decade ago. It saw big patches of bright, salty minerals. It also saw mountains, including one that’s three miles high; if you scaled Ceres to the size of Earth, the mountain would be 40 miles high. And the craft discovered that much of the surface consists of minerals that formed in a wet environment. So Ceres has water, heat, and organic compounds – the basic ingredients for life in what looks like an unfriendly world. Ceres is at a point called opposition – it lines up opposite the Sun in our sky. That means it rises around sunset and is in view all night. It’s also closest to us at opposition, so it shines at its brightest. Even so, you need binoculars or a telescope to pick it out, in the constellation Cetus. Script by Damond Benningfield

The constellations are well armed. Several of the star patterns that depict people or gods are carrying weapons. And some of them are in good view at this time of year. As darkness falls, look low in the west for the brilliant star Arcturus. It stands at the base of Boštes the herdsman. Like many of the ancient star figures, Boštes has different stories, and is drawn in different ways. In most depictions, he’s holding something long and straight against his right side. In some cases, it’s a staff. But in others, it’s a spear. Well above Boštes is Hercules, marked by a lopsided box of four stars. He’s wrestling the multi-headed hydra. And in some depictions, he’s holding up a club. In the south, look for Sagittarius. To modern eyes, it forms the outline of a teapot. But to the ancients, those stars formed an archer. The star at the outer edge of the spout is the point where he’s gripping both bow and arrow. And low in the northeast there’s a figure with a unique weapon. Perseus the hero is holding the head of Medusa. In mythology, anyone looking at Medusa was turned to stone. Perseus managed to sever the head, then used it to save the princess Andromeda from a monster. And if you’re stargazing before dawn, there’s another armed figure, well up in the south: Orion the hunter. He has two weapons. He’s holding a club in an upraised arm, with a sword strapped to his belt – a heavily armed figure in the stars. Script by Damond Benningfield

The star Fomalhaut is a bit of a disappointment. Almost two decades ago, astronomers announced the discovery of a giant planet orbiting the star – the first exoplanet actually seen at visible wavelengths of light. Almost from the beginning, though, other astronomers questioned the discovery. And they were right. It wasn’t a planet at all, but a big clump of dust – the aftermath of a giant collision. Fomalhaut is about twice as big and heavy as the Sun, and quite a bit brighter. It’s encircled by wide bands of dust. Most of the dust is at least a hundred times the distance from Earth to the Sun. Fomalhaut is only about one-tenth the age of the Sun. Even so, it’s old enough that it should have blown away most of the dust. The fact that the belts are so prominent – especially the outer belt – means that they’re being renewed. The most likely source is collisions between large comets or asteroids. As those bodies are destroyed, they spew dust out into space. One estimate says it would take the destruction of 2,000 comets that are one kilometer in diameter every day to keep the belts going. The would-be planet was the result of a collision between two even larger objects – briefly creating the illusion of a giant planet around this bright star. Fomalhaut is low in the southeast at nightfall, and climbs across the south later on. Script by Damond Benningfield

The southern evening sky is pretty bare at this time of year – lots of dark, empty spaces, but few bright stars. The one notable exception is Fomalhaut. It’s the brightest star of Piscis Austrinus, the southern fish. It’s low in the southeast at nightfall, and arcs across the south later on. The star we see as Fomalhaut is 25 light-years away. It’s about twice as big and heavy as the Sun, and more than 15 times brighter. It’s young – about 10 percent the age of the Sun. And it’s encircled by wide bands of dust, which may contain planets; more about that tomorrow. Fomalhaut has two companion stars – bound to it by their mutual gravitational pull. Both stars are smaller, cooler, and fainter than the Sun. One of them is barely visible to the eye alone, but you need a telescope to see the other. Both stars are a long way from Fomalhaut itself. One is almost a light-year away, while the other is two and a half light-years. Astronomers know they’re bound to Fomalhaut because they’re moving in the same direction and at the same speed. Their composition is similar to Fomalhaut’s as well, and so is their age. Fomalhaut itself will shine for another few hundred million years. But the companions will last much longer – billions of years for the larger one, and hundreds of billions of years for the other. So they’ll still be shining across the galaxy long after the demise of their showy companion. Script by Damond Benningfield

The Andromeda Galaxy, M31, is encircled by dozens of satellites – smaller galaxies in orbit around it. One of the larger satellites is something of an oddball. Of the three-dozen brightest, it’s the only one that lines up on the far side of Andromeda as seen from our home galaxy, the Milky Way. M31 is the closest giant galaxy to the Milky Way – just two-and-a-half million light-years away. Messier 110 is a couple of hundred thousand light-years farther. It’s a few thousand light-years in diameter, and contains about 10 billion stars – a tiny fraction the size of Andromeda. Astronomers have spent years watching M31’s entourage with Hubble Space Telescope. They recently reported that 36 of the 37 brightest members line up on the side of M31 that faces the Milky Way. And that’s hard to explain. The study said there’s only a tiny chance that the alignment is a coincidence – there must be a reason for it. But no one knows what that reason might be. It’s not a result of the Milky Way’s gravitational pull – it’s not strong enough. So there’s no obvious explanation for why M110 is an oddball – lurking on the far side of M31. M31 is low in the northeast at nightfall. Under dark skies, it looks like a hazy slash of light about as wide as the Moon. Through a small telescope, M110 looks like a bright star close by. Script by Damond Benningfield

Earth has only one moon – one large natural satellite. But it might travel with an entourage of Moon chips – bits of the Moon blasted into space by impacts with asteroids. Some of the chips may share Earth’s orbit around the Sun. Others become “quasi”-moons. They weave around the Sun in a way that looks like they’re orbiting Earth. Astronomers have catalogued a dozen or more quasi-moons in recent years. The smallest is the size of a house. The largest is about three miles across. A recent study looked at how easy it would be to make a quasi-moon as the result of an impact. The study team simulated tens of thousands of impacts across the entire Moon, at different lunar phases and with different ejection speeds. The results showed that it’s pretty darned easy. Almost seven percent of the simulations produced objects that share Earth’s orbit. And two percent became quasi-moons. They can remain in stable orbit near Earth for thousands of years before they’re kicked away. A Chinese spacecraft is scheduled to visit one of the quasi-moons next year. It’ll collect a few ounces of dirt and pebbles and return them to Earth for study. That should tell us whether the object is a chip off the ol’ Moon, or an interloper from elsewhere in the solar system. The Moon has a bright companion tonight: Antares, the brightest star of Scorpius. It’s close to the right of the Moon as they drop down the western sky in early evening. Script by Damond Benningfield

The closest giant galaxy to the Milky Way is Messier 31, the Andromeda Galaxy. It’s two-and-a-half million light-years away. But it’s getting closer – by about 250,000 miles every hour. For more than a decade, in fact, it’s looked like the two galaxies were on a collision course. But a recent study says there’s only a 50-50 chance of a collision and merger. And if it does happen, it’ll take place billions of years later than previous estimates. The new study used years of observations by two space telescopes – Hubble and Gaia. Researchers plugged those observations into simulations that also considered the gravitational effects of two smaller galaxies. The results indicated that one of them tends to push Andromeda and the Milky Way together, while the other tends to pull them apart. The researchers ran a hundred thousand simulations. In half of them, Andromeda and the Milky Way flew past each other and went their own ways. In the other half, they eventually spiraled together and merged – but not for at least 10 billion years – twice as long as earlier estimates. The simulations aren’t the final word – there are just too many uncertainties. But for now, it seems likely that the two giants will stay apart for a long, long time. M31 is in the northeast at nightfall. Under dark skies, it’s visible as a hazy patch of light. Binoculars make it easier to pick out. Script by Damond Benningfield

Messier 31, the Andromeda Galaxy, is the largest and most-distant object that’s easily visible to the unaided eye. Under dark skies, it looks like a skinny cloud about as wide as the Moon. Right now, it’s about a third of the way up in the northeast at nightfall. M31 is two-and-a-half million light-years away. In other words, the light you see from the galaxy tonight began its journey across the cosmos two-and-a-half million years ago. The galaxy is roughly 150,000 light-years across – bigger than the Milky Way – and may contain a trillion stars. It’s also the hub of its own galactic empire – it’s orbited by more than three dozen smaller galaxies. And a recent study revealed many new details about the satellites. Astronomers spent years looking at them with Hubble Space Telescope. And they supplemented the new observations by going through older ones. They found that most of the stars in the smaller galaxies had been born by about 12 billion years ago – when the universe was about one-tenth of its present age. And star formation had all but stopped by about eight billion years ago. Galaxies that are bigger and farther from M31 gave birth to stars a little longer than those that are small and close. One of the bigger satellites might have rammed through M31 a few million years ago. That stirred things up throughout the empire surrounding big, beautiful M31. More about M31 tomorrow. Script by Damond Benningfield

People become astronomers for many reasons: They’re interested in the workings of the stars, or the quest to find life in the universe, or the fate of the universe itself. Geoffrey Burbidge joked that he became an astronomer because he married one. He and his wife, Margaret, were astronomy’s power couple. And they co-authored one of the most important studies of the 20th century. Burbidge was born 100 years ago today, in the English village of Chipping Norton. He first studied history, but switched to physics. After earning his undergraduate degree, shortly after World War II, he developed bombs for a while. Back in academia, he married Margaret, and they hopped around England and the United States over the next few decades. Burbidge contributed to many areas of astronomy theory. But he’s best known for a single paper, known as B-squared-F-H for the names of its authors – the two Burbidges, William Fowler, and Fred Hoyle. In it, they explained how stars forge most of the elements in the universe. Many elements are created in a star’s core during its long life; others, in the violent deaths of stars. Some of the elements are expelled into space, where they can be incorporated into new stars. The newer generations make even more elements – eventually creating the chemistry we see in the universe today. So the paper showed that we’re all made of “starstuff” – elements created in the stars. Script by Damond Benningfield

Neptune is one of the giants of the solar system. But it’s so far away that it’s tough to study. We know little about its interior. And much of what scientists think they know comes from lab experiments and computer models. Neptune is the Sun’s most remote major planet. So although it’s almost four times Earth’s diameter, it’s a tiny target for telescopes. And only one spacecraft has ever visited the planet – Voyager 2, in 1989. From those observations, along with those from telescopes on the ground and in space, scientists have developed a model of how Neptune is put together. It probably has a dense, rocky core, surrounded by an “ocean” of super-heated water, ammonia, and methane. The pressure squeezes this layer so tightly that the compounds act like ice. Around that is a layer of hydrogen, which is topped by a methane-rich atmosphere. The methane absorbs red light, giving the planet a blue-green color. It’ll be decades before another mission can approach Neptune. Until then, we’ll have to rely on a lot more calculations to understand this remote giant. Neptune is at its best right now. It’s in view all night and it’s brightest for the year. Even so, you need a telescope to spot it. But you can easily spot its location. As night falls, look for Saturn, which looks like a bright star, low in the east. Neptune is to the left of Saturn, by a bit more than a finger held at arm’s length. Script by Damond Benningfield

Earth “falls” into a new season today – astronomically speaking. It’s the September equinox, when the Sun crosses the equator from north to south. It marks the start of autumn in the northern hemisphere, and spring in the southern hemisphere. On the equinoxes, neither the north pole nor the south pole tips toward the Sun, so night and day are roughly the same length in both hemispheres – about 12 hours between sunrise and sunset. We say “roughly” because there are a couple of caveats. One is the way we calculate the times of sunrise and sunset. For the days to be truly equal, we’d have to mark the times when the Sun is bisected by the horizon – half in view, half still hidden. But we don’t. Instead, sunrise is the moment when the Sun first peeks into view, and sunset is the moment when the limb of the Sun drops from view. That adds a couple of minutes to the day. The other correction factor is Earth’s atmosphere. It “bends” the sunlight above the horizon. So when we see the Sun standing just atop the horizon, it’s actually a little below it. That combination adds a few minutes to the equinox “day.” So at the equator, daylight lasts for 12 hours plus six and a half minutes. At 30 degrees north – the latitude of Austin – it’s 12 hours and eight minutes. And at 60 degrees – roughly the latitude of Anchorage – it’s 12 hours and 16 minutes – an extra dose of sunlight as we fall into autumn. Script by Damond Benningfield

Officially, Saturn has 274 known moons. Un-officially, it has billions upon billions of them – the bits of ice and rock that make up the planet’s rings. They range from the size of dust grains to giant boulders. All of them orbit the giant planet like tiny moons. The system consists of three main bands, which are easy to see. Together, they span about three-quarters of the distance between Earth and the Moon. But there are some thinner, fainter bands as well. One is closer to Saturn than the main bands, while the others are farther. Despite their great span, the rings are quite thin – generally no more than a few dozen feet thick. Individual rings are held in check by the gravity of some of Saturn’s moons and “moonlets” – bodies no more than a few hundred feet in diameter that orbit inside the ring system. In some cases, they force the rings to intertwine like the braids in a loaf of challah bread. Scientists are still debating the age of the rings. Estimates range from a hundred million years to more than four billion. Either way, the rings are constantly replenished with fresh supplies of ice and dust – sustaining one of the most amazing features in the solar system. Saturn is at its best for the entire year. It looks like a bright star, low in the east at nightfall and climbing high across the south during the night. Telescopes reveal the planet’s beautiful rings. Script by Damond Benningfield

Happy Saturn’s Day – the day of the week named for Saturn, the second-largest planet in the solar system. And the name is especially fitting today, because the planet is at its best for the entire year. It looks like a bright star, shining all night long. The seven-day week was created in ancient Babylon. The days were named for the seven known “planets.” The list included the Sun, Moon, and the five true planets that are easily visible to the naked eye. The day was split into 24 hours, with each hour named for a planet. The planets were ranked by how long it took them to cross through the background of stars. Saturn takes the longest, so it was number one on the list. Each day was named for the planet that came up in the first hour of that day. So “Saturn’s Day” was the first day of the week. That changed later on, especially in the Christian era, when the week began a day later, on the Sun’s Day – Sunday. Other than Saturn and the Sun and Moon- Saturday, Sunday, and Monday – old English adopted the planet names from the Norse pantheon of gods. Tuesday is named for Tiw – the representation of Mars. It’s followed by Woden, Thor, and Freya – Mercury, Jupiter, and Venus – celestial names for the days of the week. Saturn is low in the east at nightfall, and looks like a bright star. It climbs high across the south later on, and sets around sunrise. We’ll talk about Saturn’s rings tomorrow. Script by Damond Benningfield

These are the sounds of Mars: a dust devil … a rover trundling across the surface … the steady sigh of the wind. All of these sounds were recorded by the Perseverance rover – the first craft to carry microphones to Mars. Scientists have used the recordings to learn more about how sound carries on Mars. The planet’s atmosphere is less than one percent as thick as Earth’s atmosphere, so it’s much quieter on Mars. It’s especially quiet around noon, when sound waves are bent upward, away from the ground. The atmosphere is also much colder than Earth’s, and it’s made mainly of carbon dioxide. Combined with the air’s low density, on average, sound travels about 30 percent slower on Mars. And there’s a big difference in both the speed and distance at which different frequencies travel. Higher frequencies die out more quickly, and they move slower. So if you wanted to carry on a conversation – if you could survive without a spacesuit, of course – you’d want a nice, deep voice. Mars is disappearing in the evening twilight. From the northern part of the country, in fact, it’s probably too low in the twilight to see at all. From south of about Dallas, it looks like a moderately bright star quite low in the west-southwest as twilight begins to fade – silently dropping from sight. Script by Damond Benningfield

There’s an extraordinary conjunction in tomorrow’s early morning sky – a tight grouping of the Moon, the planet Venus, and the star Regulus. They’re quite low at first light, so you may need a clear horizon to spot them. Venus is the brilliant “morning star,” just a fraction of a degree from the Moon. Regulus is a bit farther from the Moon. It’s much fainter than Venus, but its proximity to the brighter bodies will make it pretty easy to pick out. This beautiful meeting is possible because all three bodies lie near the ecliptic – the Sun’s path across the sky. Regulus, which marks the heart of the lion, is “fixed” in position just half degree a from the ecliptic. It does move through the galaxy, but it’s so far away that it takes centuries to notice any change. Venus’s orbit around the Sun is tilted by about three degrees – about one and a half times the width of your finger held at arm’s length. The planet crosses the ecliptic during each orbit, so it’s always close. On rare occasions, it can even cross in front of Regulus, blocking it from view. That last happened in 1959, and it’ll happen again on October 1st, 2044. The Moon’s orbit around Earth is tilted by about five degrees. So, like Venus, the Moon moves back and forth across the ecliptic. Tomorrow, it’ll be just about one degree from that path – setting up a beautiful conjunction in the dawn sky. Script by Damond Benningfield

A third of a century ago, we knew of only two solar-system bodies beyond the orbit of Neptune: Pluto and its largest moon. Today, the known population of such bodies is in the thousands. And quite a few of them are in the same class as Pluto itself: dwarf planets. One of the newest members of that class is 2017 OF201. It was discovered in 2017. A recent study found that it may be about a third the size of Pluto. If so, then it most likely would qualify as a dwarf planet. The object follows a highly elongated orbit around the Sun. It ranges from about 45 times to 1600 times the distance from Earth to the Sun. And it takes almost 25,000 years to complete a single orbit. Today, it’s about 90 times the Earth-Sun distance, and moving outward. Before long, it’ll be so remote that not even the biggest telescopes can see it. Researchers say the object could be bad news for a possible Planet Nine. Studies of other objects in the outer solar system suggest that some of them may have been pushed around by the gravity of a much larger body. That body could be a planet roughly five to 10 times the mass of Earth, orbiting far from the Sun. But the orbit of 2017 OF201 shows no influence of such a planet. There’s a lot to be done to understand the orbits of the bodies in the outer solar system – and use them to pinpoint a possible planet far from the Sun. Script by Damond Benningfield

The roster of “dwarf planets” keeps growing. But it’s not official – there’s no league office to tell us who’s on the roster and who’s not. Various groups keep their own lists, but they don’t agree on which objects belong. The dwarf-planet category was formalized a couple of decades ago. Astronomers had discovered some new Pluto-like objects beyond the orbit of Neptune. They had to decide whether to add those objects to the roster of planets, or to put them in a new category. So in 2006, the International Astronomical Union voted to create the “dwarf planet” designation. A dwarf planet was defined as a body that’s large enough for its gravity to pull it into a rounded shape, but not large enough to clear its orbit of other bodies. The initial list included Pluto and three other distant objects, plus Ceres, the largest asteroid. Since then, astronomers have discovered thousands more objects in the realm of Pluto and beyond. Most of them are fairly small. But some are larger. Because they’re so far away, though, it can be tough to figure out an exact size and mass. So that makes it harder to decide whether some of these bodies are dwarf planets, or just big comets or asteroids. Today, most planetary scientists agree on a core list of about 10 dwarf planets. Another dozen or so are considered good candidates. And many more are possibilities – including a recently discovered one that we’ll talk about tomorrow. Script by Damond Benningfield

A bright star and planet team up with the Moon early tomorrow to form a tight, beautiful triangle. Pollux will stand close to the lower left of the Moon, with much brighter Jupiter about the same distance to the lower right of the Moon. Pollux is the brightest star of Gemini, while Jupiter is a planet. Jupiter is by far the giant of the solar system. It’s more than twice as massive as all the other planets combined. And it’s about 11 times the diameter of Earth. That makes it big enough to hold 1300 Earths. But a recent study says that Jupiter might have been much bigger during its infancy – about two or two-and-a-half times its current diameter. That would have made it big enough to hold thousands of Earths. Scientists came to that conclusion by studying the orbits of two of Jupiter’s small, close-in moons. The orbits are slightly tilted. Simulations showed that the moons were pushed into those orbits by the larger moon Io as it moved away from Jupiter. Those calculations revealed Jupiter’s original size and other details. Jupiter probably formed in just a few million years – much quicker than most of the other planets. By then, the supply of planet-making materials had dried up. So Jupiter’s gravity began squeezing it and making it spin faster. Eventually, the planet reached a point where it couldn’t shrink any farther – leaving the smaller but still-giant world we see today. Script by Damond Benningfield

A star that may be in a death spiral wants the universe to know about it. Every four and a half days it creates a burst of X-rays. The cause of those outbursts may be leading to the star’s demise. The possibly dying star is in a galaxy that’s about 300 million light-years away. During evening twilight now, that spot is quite low in the west, below the bright star Arcturus. According to a recent study, the story probably involves the star; a black hole, nicknamed Ansky, that’s a million times the mass of the Sun; and a wide disk of hot gas around the black hole. The star is following a tilted orbit around the black hole. Every few days, the star plunges through the disk. That heats the gas around the star, so gas blows away from the disk in bubbles that may be as massive as the planet Jupiter. Each passage robs the star of a bit of its orbital energy, so it spirals closer to the black hole. If the star is the mass of the Sun, it could last another five or six years before it dives into the black hole or is ripped apart by the black hole’s gravity. If the star is heavier, it could survive a little longer. Astronomers discovered the system in observations by two X-ray telescopes in space. They’ll use those same telescopes to watch the system in the years ahead. If the outbursts get more frequent, it’ll confirm they’re on the right track, and the star is on the wrong one – headed toward its destruction. Script by Damond Benningfield

There’s a season for everything, from football to Broadway to allergies. There are seasons in the heavens as well. And the next act in one of those seasons plays out early tomorrow: an occultation by the Moon of the star Elnath – the tip of one of the horns of Taurus. An occultation takes place when one object covers up another. The Moon occults a few fairly bright stars every month. And the occultations occur in seasons. That’s because the Moon’s orbit is tilted with respect to the ecliptic – the Sun’s path across the sky. The Moon moves back and forth across the ecliptic, allowing it to occult any star within a few degrees of that path. But its position relative to any particular star changes from year to year. As a result, occultations occur in bunches – in seasons. Now, the Moon is in the middle of its occultation season with Elnath. The season began in 2023, and continues into 2027. Because of the different angles to the Moon and star, and the short length of each event, only a few of the occultations are visible from a particular location. This occultation will be visible from the far-southwestern United States. Elnath and the Moon rise into good view after midnight, with the star to the lower left of the Moon. The Moon will slip toward Elnath as they climb higher. From most of the country, the Moon and star will just miss each other – a “seasonal” encounter in the dawn sky. Script by Damond Benningfield

The Milky Way is packed with star clusters – thousands of them. They contain anywhere from a few dozen stars to more than a million. And the most impressive of them all is right in the middle – it surrounds the supermassive black hole at the heart of the galaxy. The Nuclear Star Cluster contains up to 10 million stars. They extend a couple of dozen light-years from the black hole in every direction. But most of them are packed in close. If our part of the galaxy were that densely settled, we’d have a million stars closer to us than our current closest neighbor, Alpha Centauri. So any planets in the cluster would never see a dark night. Most of the stars in the cluster formed about 10 billion years ago, when the galaxy was young. But there was another wave of starbirth about three billion years ago, and a smaller one just a hundred million years ago. Each wave might have been triggered when the Milky Way swallowed a smaller galaxy. As the galaxies merged, clouds of gas and dust settled in the middle, around the Milky Way’s black hole. That gave birth to new stars – populating the galaxy’s most impressive cluster. The cluster is in Sagittarius, which is due south at nightfall. The constellation looks like a teapot. The center of the galaxy is in the “steam” rising from the spout. But giant clouds of dust absorb the light from the galaxy’s heart, so it takes special instruments to see the cluster. Script by Damond Benningfield

Guillaume Le Gentil spent more than 11 years away from his native France just to witness two brief astronomical events. Along the way, he had to survive war, a hurricane, disease, and grumpy officials. When he got home, he’d lost his job and been declared dead. But the real hardship? He missed both events. Le Gentil was born 300 years ago this week. He studied theology, but decided on astronomy as a career. He became a member of the Royal Academy of Science at age 28. Le Gentil and other astronomers hoped to measure a 1761 transit of Venus across the Sun from many locations on Earth. The details would reveal the Sun’s distance – the basic “yardstick” for the entire solar system. Le Gentil planned to watch from India. He headed out in March of 1760. War with England complicated the trip, and his ship was blown off course. On the day of the transit he was still at sea, where it was impossible to make observations. The next transit was just eight years away, so Le Gentil decided to hang around. He planned to watch from the Philippines. But he got a chilly reception, so he returned to India. He set up an observatory and waited. But the day of the transit was cloudy – until shortly after it was over. Heartbroken, Le Gentil headed home. It took two hard years to get there – only to encounter even more problems. But he worked things out, and published two volumes about his travels in the name of science. Script by Damond Benningfield

If you’d like to travel into the future – even the far-distant future – you don’t need a time machine. Instead, a starship will do just fine. Fire up the engines, head into space, and keep your foot on the gas. The laws of physics seem to make it impossible – or nearly so – to travel through time in anything like the modern concept of a time machine – something that allows you to move through the centuries at will. Yet those same laws make it possible to zoom into the future. The concept is known as time dilation. As you travel faster, your clock ticks more slowly compared to the clocks of those you left behind. It’s been proven by putting atomic clocks in airplanes and aboard GPS satellites. In fact, if GPS clocks weren’t adjusted to account for it, the entire system would fail. At the speed of a satellite, the difference is tiny – a few millionths of a second per day. As speed increases, though, the effect becomes more significant. If you could travel at 90 percent of the speed of light for one year as measured by the clock on your ship, more than two-and-a-quarter years would pass back on Earth. At 99 percent of lightspeed, it’s more than seven years per ship year. And at 99.99 percent, the ratio is 70 Earth years per ship year. Of course, there is the problem of finding a fast starship to carry you. But so far, that’s the only known way to beat Time – and travel into the future. Script by Damond Benningfield

Based on the number of books, movies, and TV shows about it, you might assume that traveling through time is almost as easy as ambling through the park on a sunny day: Just build a TARDIS or soup up your Delorean, and off you go. Alas, the arrow of time moves in only one direction. It allows you to travel into the future, but roadblocks seem to prevent any method that scientists can envision for traveling in the other direction. Wormholes, for example, are theoretical “tunnels” through space and time. They seem to allow travel to other times – past or future. But there’s a problem: The wormhole may collapse as soon as anything enters it – a person, a spaceship, or even a radio beam. Another possibility for traveling into the past is moving really fast. Albert Einstein’s theories of relativity suggest that anything moving faster than light might move backward in time. But any physical object moving at lightspeed would become infinitely massive. That means you’d need an infinite amount of energy just to reach lightspeed – and even more to go faster. A few decades ago, Stephen Hawking suggested that the universe doesn’t like time travel. He wrote that the laws of physics may stop anyone from ever building a time machine – keeping the past safe from its own future. Even so, physics provides some tricks that allow travel to the future, and we’ll have more about that tomorrow. Script by Damond Benningfield

If a cosmic giant sat on a big, gassy planet, it would look a lot like Saturn, the second-largest planet in the solar system. It’s 10 percent wider through its equator than through the poles. But Saturn flattened itself – a result of its low density and fast rotation. Saturn consists of a series of layers. Its core is a dense ball of metal and rock. Around that is a layer of hydrogen that’s squeezed so tightly that it forms a metal. Around that is a layer of liquid hydrogen – the lightest and simplest chemical element. And the planet is topped by an atmosphere that contains methane, ammonia, water, and other compounds. Despite its great size, Saturn spins once every 10.7 hours. That pushes material outward, making the planet fatter through the equator. The combination of its composition and rotation makes Saturn especially light – it’s less dense than water. Saturn doesn’t have a solid surface. But scientists have defined a “surface” as the depth in its atmosphere where the pressure equals the surface pressure on Earth. At that level, Saturn’s gravity is only a bit stronger than Earth’s gravity. So if you were floating at that altitude, you’d feel like you’d added a few pounds. And because of Saturn’s flattened shape, you’d feel heavier at the poles than the equator. Look for Saturn near the Moon tonight. It looks like a bright star to the right of the Moon in early evening, and farther below the Moon at dawn. Script by Damond Benningfield

Building the planets of the solar system was like building a city – it didn’t happen all at once. Instead, it probably took a hundred million years or more to complete the construction project. The first to be completed were Jupiter and Saturn, the Sun’s largest planets. They came together in the prime real estate for planet building – the region with the most raw materials. Closer to the Sun, it was so hot that ices were vaporized and blown away. Farther from the Sun, the material thinned out. But at the distance of Jupiter and Saturn, the balance was just right. The two giants took shape in a hurry. Small grains of ice and rock stuck together to make pebbles, then baseball-sized chunks, then boulders, and so on. That quickly built massive cores, which then swept up huge amounts of leftover hydrogen and helium gas. So within just a few million years, Jupiter and Saturn have grown to monstrous proportions. Uranus and Neptune took shape a little later – within tens of millions of years. Earth and the other rocky inner planets took a bit longer – at least a hundred million years. So the biggest planets of the solar system are also the oldest – dating to shortly after the birth of the Sun. Saturn stands close to the Moon the next couple of nights. The planet looks like a bright star. It’s to the lower left of the Moon as darkness falls tonight, and about the same distance to the right of the Moon tomorrow night. Script by Damond Benningfield

The 41st episode of a celestial series plays out tomorrow: a total lunar eclipse. It’ll be visible around much of the world – but not the Americas. Every eclipse belongs to a series, called a Saros. The eclipses in a Saros are separated by 18 years plus 11 and a third days. If we could watch all the eclipses in the cycle play out, we’d see the Moon pass through Earth’s shadow from top to bottom or bottom to top. So the Moon barely dips its toe in the shadow at the beginning and end of the sequence. But it’s fully immersed during the middle of the cycle, creating total eclipses. And because of that extra third of a day in the cycle, each eclipse occurs a third of the way around the world from the previous one. This eclipse is part of Saros 128. The cycle began in 1304 and will end in 2566 – 71 eclipses in all. Most of Asia and Australia will see this entire eclipse, from beginning to end. And most of the rest of the world will see at least part of it. Totality – when the Moon is completely immersed in the shadow – will last for an hour and 22 minutes. But the eclipse occurs during the middle of the day for those of us in the United States, so we won’t see any of it. What we will see the next couple of nights, though, is a beautiful full Moon – the Fruit Moon or Green Corn Moon – completely free of Earth’s dark shadow. Script by Damond Benningfield

The Moon will briefly cover up the tail of the sea-goat tonight – Deneb Algedi, the brightest star of Capricornus. The sequence will be visible across much of the United States. This vanishing act is an occultation – a type of eclipse in which one object completely covers another. But eclipses are nothing new for Deneb Algedi. Not only does it periodically get covered up by the Moon, but it stages its own eclipses – two of them every day. What we see as Deneb Algedi is a binary – two stars in a tight orbit around each other. The main star in the system is about twice as big and heavy as the Sun, and much brighter. Its companion is a little smaller and fainter than the Sun. We’re looking at the system edge-on, so the stars pass in front of each other – creating eclipses. When the fainter star crosses in front of the brighter one, the system’s overall brightness drops by about 20 percent – enough for a skilled skywatcher to notice. But when the brighter star eclipses the fainter one, the dip is much smaller, so it’s detectable mainly with instruments. The stars orbit each other once a day. That means we see two eclipses per day – just 12 hours apart. Deneb Algedi isn’t especially bright, so it’s hard to see through the bright moonlight. But binoculars will help you pick it out. From much of the western U.S., the Moon will just miss the eclipse-happy tail of the sea-goat. Script by Damond Benningfield

The planet Jupiter will slide past one of the brighter stars of Gemini the next few mornings. At their closest, they’ll be separated by just a fraction of a degree. The star is Wasat – from an Arabic phrase that means “the middle.” But the middle of what has been lost over the centuries. The star also is known as Delta Geminorum – its Bayer designation. The system was devised in the early 17th century by German astronomer Johann Bayer. He named all of the stars in the constellations that were visible from the northern hemisphere. Each star was given a Greek letter followed by the constellation name. If he ran out of letters, he switched to the Latin alphabet. In most constellations, Bayer named the stars in the order of their brightness. The brightest was alpha, the next-brightest was beta, and so on. Sometimes, he ranked the stars on their location or some other system. And he named the stars based on how they looked to the naked eye, so the rankings were completely subjective. So even though delta is the fourth letter in the Greek alphabet, Delta Geminorum is only the eighth-brightest star in Gemini. Jupiter and Wasat are well up in the east at dawn. Jupiter looks like a brilliant star, far to the upper right of even-brighter Venus. Wasat will stand below Jupiter tomorrow. Jupiter will drop past it over the following couple of days, so they’ll be at their closest on Saturday and Sunday. Script by Damond Benningfield

Water is the key ingredient for life on Earth. And as far as we know, it’s a key ingredient for life everywhere else in the universe as well. That shouldn’t be a problem, though, because there’s plenty of water to go around. Water is common in part because it’s made of two of the three most common elements in the universe – hydrogen and oxygen. They come together in the cold of deep space to make grains of ice. Some of those grains are found in the clouds of gas and dust that give birth to new stars and planets. Others form inside those clouds. In recent years, astronomers have found evidence of water in other star systems, and even in other galaxies. They’ve found grains of ice in the disks of material around newborn stars. They’ve seen giant belts of comets, which contain a lot of ice. They’ve discovered water vapor in the atmospheres of a few planets. And they’ve even found evidence that some planets could be covered in oceans of liquid water. One example is TOI 1452 b, which orbits a star that’s much smaller and fainter than the Sun. The planet itself is bigger and heavier than Earth. Given its details and its distance from the star, scientists say it could have a deep global ocean – a possible home for life. TOI 1452 is about a hundred light-years away, in Draco. The dragon twists high across the north at nightfall. But the star is much too faint to see without a telescope. Script by Damond Benningfield

Earth is the only body in the solar system with liquid water on its surface. But it’s not the only one where you can find water. In fact, water is everywhere – from comets and asteroids to the giant planets. Comets and asteroids are chunks of rock, metal, and ices – including water ice. Comets have more ice, but most asteroids probably have large amounts as well. Such bodies might have supplied much of the water on Earth when they collided with our planet billions of years ago. Water ice is common throughout the solar system. It’s been seen at the poles of the Moon and the planet Mercury – the Sun’s closest planet. It forms large polar caps on Mars. And it coats many of the moons of the giant outer planets. Water also has been detected in the clouds of Jupiter and Saturn. And it may be a major component of the outer layers of Uranus and Neptune, the Sun’s most remote planets. To find liquid water, you have to go deep. There may be global oceans of water far below the surfaces of some of the big moons of Jupiter and Saturn, and perhaps some moons of Uranus and Neptune as well. Some of those oceans could hold more water than all of Earth’s oceans combined. On Earth, water is a key ingredient for life. So some of the moons of the outer planets are considered good places to look for life – swimming in oceans of liquid water. We’ll talk about water beyond the solar system tomorrow. Script by Damond Benningfield

Water is all about extremes. The atoms that make up water molecules were forged in some of the hottest environments in the universe. But most of the molecules formed in the cold of deep space. A water molecule consists of two hydrogen atoms plus one oxygen atom – H-2-O. A hydrogen atom contains one electron and one proton. The electrons formed in the first fraction of a second after the Big Bang, when the universe was extremely hot and dense. The protons formed a few minutes later. By about 380,000 years, the universe had expanded and cooled enough for the electrons and protons to stick together to form atoms. And today, hydrogen accounts for more than 90 percent of all the atoms in the universe. Hydrogen and helium, the other major element forged in the Big Bang, soon came together to make stars. And a star’s core is hot enough to “fuse” lighter elements to create heavier ones. The first steps in that process create carbon, nitrogen, and especially oxygen – the third-most abundant element in the universe. When stars die, they expel some of those elements into space. And in the cold away from the stars, hydrogen and oxygen can stick together to make molecules of water. Some of the water’s incorporated into planets – including our own. So the next time you take a cool drink of water, think of the hot-and-cold origins of this important compound. We’ll have more about water tomorrow. Script by Damond Benningfield